Advanced OSPF Design Concepts

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Usage Guidelines: This command is used only on an area border router attached to a stub area. There are two stub area router configuration commands: the stub and default-cost options of the area command. In all routers and access servers attached to the stub area, the area should be configured as a stub area using the stub option of the area command. Use the default-cost option only on an ABR attached to the stub area. The default-cost option provides the metric for the summary default route generated by the ABR into the stub area.

Example: The following example assigns a default-cost of 20 to stub network 36.0.0.0:

    interface ethernet 0    ip address 36.56.0.201 255.255.0.0    !    router ospf 201    network 36.0.0.0 0.255.255.255 area 36.0.0.0    area 36.0.0.0 stub    area 36.0.0.0 default-cost 20 

Related Commands: area authentication, area stub

area-id

area-id refers to the area that is to be associated with the OSPF address range. It can be specified as either a decimal value or as an IP address. If you intend to associate areas with IP subnets, you can specify a subnet address as the area-id.

Default: Disabled.

Command Mode: Router configuration.

Usage Guidelines: The address and wildcard-mask arguments together enable you to define one or multiple interfaces to be associated with a specific OSPF area using a single command. Using the wildcard-mask enables you to define one or multiple interfaces to be associated with a specific OSPF area using a single command. If you intend to associate areas with IP subnets, you can specify a subnet address as the area-id.

The Cisco IOS software sequentially evaluates the address/wildcard-mask pair for each interface as follows:

1.  The wildcard-mask is enabled logically with the interface IP address.
2.  The wildcard-mask is enabled logically with the address in the network command.
3.  The software compares the two resulting values.
4.  If they match, OSPF is enabled on the associated interface logically and this interface is attached to the OSPF area specified.


Notes:  
Any individual interface can only be attached to a single area. If the address ranges specified for different areas overlap, the software will adopt the first area in the network command list and ignore the subsequent overlapping portions. In general, it is recommended that you devise address ranges that do not overlap to avoid inadvertent conflicts.

Example: In the following partial example, OSPF routing process 109 is initialized, and four OSPF areas are defined: 10.9.50.0, 2, 3, and 0. Areas 10.9.50.0, 2, and 3 mask specific address ranges, while area 0 enables OSPF for all other networks.

    router ospf 109    network 131.108.20.0 0.0.0.255 area 10.9.50.0    network 131.108.0.0 0.0.255.255 area 2    network 131.109.10.0 0.0.0.255 area 3    network 0.0.0.0 255.255.255.255 area 0 

Related Commands: router ospf

area nssa

To configure an area as a not-so-stubby area (NSSA), use the area nssa router configuration command. To remove the nssa distinction from the area, use the no form of this command. The syntax for this command (and the no form) is as follows:

    area area-id nssa [no-redistribution] [default-information-originate]    no area area-id nssa 

Syntax Description:

area-id. Identifier of the area for which authentication is to be enabled. The identifier can be specified as either a decimal value or an IP address.
no-redistribution (optional). This is used when the router is a NSSA ABR and you want the redistribute command to import routes only into the normal areas, but not into the NSSA area.
default-information-originate (optional). Used to generate a Type 7 default into the NSSA area. This command only takes effect on NSSA ABR.

area range

To consolidate and summarize routes at an area boundary, use the area range router configuration command. To disable this function, use the no form of this command. The syntax for this command (and the no form) is as follows:

    area area-id range address mask    no area area-id range address mask 

Syntax Description:

area-id. Identifier of the area about which routes are to be summarized. It can be specified as either a decimal value or as an IP address.
address. IP address.
mask. IP mask.

Default: Disabled.

Command Mode: Router configuration.

Usage Guidelines: The area range command is used only with area border routers. It is used to consolidate or summarize routes for an area. The result is that a single summary route is advertised to other areas by the area border router. Routing information is condensed at area boundaries. External to the area, a single route is advertised for each address range. This is called route summarization.

Multiple area router configuration commands specifying the range option can be configured. Thus, OSPF can summarize addresses for many different sets of address ranges.

Example: The following example specifies one summary route to be advertised by the ABR to other areas for all subnets on network 36.0.0.0 and for all hosts on network 192.42.110.0:

    interface ethernet 0    ip address 192.42.110.201 255.255.255.0    !    interface ethernet 1    ip address 36.56.0.201 255.255.0.0    !    router ospf 201    network 36.0.0.0 0.255.255.255 area 36.0.0.0    network 192.42.110.0 0.0.0.255 area 0    area 36.0.0.0 range 36.0.0.0 255.0.0.0    area 0 range 192.42.110.0 255.255.255.0 

area stub

To define an area as a stub area, use the area stub router configuration command. To disable this function, use the no form of this command. The syntax for this command (and the no form) is as follows:

    area area-id stub [no-summary]    no area area-id stub 

Syntax Description:

area-id. Identifier for the stub area. The identifier can be either a decimal value or an IP address.
no-summary (optional). This prevents an ABR from sending summary link advertisements into the stub area.

Default: No stub area is defined.

Command Mode: Router configuration.


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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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